This invention relates generally to the electrical interconnection of circuits, and more particularly to such interconnects which are especially adapted for making external electrical connections to thermal ink jet printheads.
It is known to provide heater resistors on a common substrate, such as silicon, and employ these resistors to transfer thermal energy to corresponding adjacent ink reservoirs during a thermal ink jet printing operation in the manufacture of thin film resistors substrates for thermal ink jet printheads. This thermal energy will cause the ink in the reservoirs to be heated to boiling and thereby be ejected through an orifice in an adjacent nozzle plate from which it is directed onto a print medium. These heater resistors are electrically pulsed during such operation by current applied thereto via conductive traces formed on top of the silicon substrates and insulated therefrom by an intermediate dielectric layer. The formation of an intermediate dielectric layer, the formation of the resistive layer for the heater resistors, and the aluminum evaporation of sputtering process for forming electrical patterns of conductive trace material to the heater resistors are all well known in the art and therefore are not described in further detail herein. The processes used in the fabrication of thermal ink jet printheads are discussed in the Hewlett Packard Journal, Volume 36, Number 5, May 1985 ("HP Journal Article"), which is incorporated herein by reference. Hewlett Packard Corporation is the assignee of the entire right, title and interest in the subject patent application.
Electrical connections are provided between external pulse drive circuits and the conductive traces on the thermal ink jet printhead using flex or "flex" circuits to make removable pressure contacts to certain conductive terminal pads on thin film resistor printhead substrates or to tape automated bonding (TAB) circuits. These electrical connections are facilitated by applying pressure to the flex circuit so that the electrical leads therein make good electrical connection with corresponding mating pads on the thin film resistor printhead substrate. These flex circuit generally comprise photolithographically defined conductive patterns formed by various etching processes carried out on a thin flex insulating substrate member. The electrical contact locations on the flex circuit will be raised slightly in a bump and dimple configuration. This configuration is formed using a punch structure which matches the location of the correspondingly dimples. The punch structure is used to form the electrical contact locations on the flex circuit at raised locations above the surface of the insulating substrate member. During this punch process, it sometimes happens that not all of the raised contact bumps in the flex circuit are moved the same distance above the insulating substrate surface thereby producing a nonuniform dimple configuration. For this reason, more force is necessary to make contact with the smaller, or lower height bumps than those higher bumps more extended from the surface of the flex circuit. When a significant force is exerted against the flex circuit by the printhead in order to interconnect same, crushing of a portion of the raised dimple structure will result. Furthermore, the presence of a nonuniform dimple configuration will prevent contact of the printhead and flex circuit at their interface.
Other problems result from the use of a dimpled configuration per se. The raised dimple structure formation process is expensive to fabricate and requires high contact forces in its implementation. Moreover, there is poor control over the point geometry of that formation process. Spacing of the dimples in the overall dimple configuration is also a problem because they need to be spaced a relatively close intervals. However, spacing is limited by the thickness and fragility of the metal employed to form the dimpled structure. The close spaced dimpled structure, which is unique to ink jet printing, is quite difficult to manufacture.
Contact between the flex circuit and conductive pads on the TAB circuit can be maintained by using an elastomeric material, such as rubber, which has been preformed to have a plurality of cones spaced at locations corresponding to the location of the dimples in the flex circuit. The tips of these elastomeric cones can be inserted into the dimples of the flex circuit and urged thereagainst with a force sufficient to bring the conductive bumps on the flex circuit in to good physical and electrical contact with the terminal pads on the TAB circuit.
A contact array (see FIG. 1 of the HP Journal Article) can be integrated with a flex printed circuit that carries the electrical drive pulses to the printhead. Connector mating is achieved by aligning the printhead cartridge registration pins with the mating holes in the carriage/interconnect assembly and then rotating a cam latch upward or pivoting the printhead into position. In this way, electrical contact can be made without lateral motion between the contact halves. The contact areas are backed with silicon-rubber pressure pads (see FIG. 2 of the HP Journal Article) which allow electrical contact to be maintained over a range of conditions and manufacturing tolerances. Electrical contact is enhanced by dimpling the flex circuit pads. The dimples are formed on the flex circuit before the plating is applied.
While the above prior art approach to making electrical contact between the flex circuit and the print-head substrate has proven satisfactory for certain types of interconnect patterns with few interconnect members, it has not been entirely satisfactory for low voltage signal contacts. This fact has been a result of the nature of the nonlinear deflection of the above elastomeric cones. This nonlinear deflection of the elastomeric cones is seen as a nonlinear variation of cone volumetric compression, "V", as a function of the distance, "D", that the tip of the cone is moved during an interconnect operation. Thus, this nonlinear characteristic tends to increase the amount of force which must be applied to the flex circuit in order to insure that all the bumps on the flex circuit make good electrical contact with the conductive traces of terminal pads on the printhead substrate. In some cases this required force is sufficiently large to fracture the substrate or do other structural damage thereto. This non-linear deflection characteristic of the prior art is described in more detail below with reference to the prior art FIGS. 1A and 1B of U.S. Pat. No. 4,706,097, which is incorporated herein by reference.
In order to reduce the amount of force required to insure good electrical contact between a flex circuit and a TAB circuit for a thermal ink jet printhead, a novel, nearly-linear spring connect structure for placing the flex circuit into good electrical contact with contact pads on the printhead substrate with a minimum of force applied thereto was developed. This structure is set forth in the U.S. Pat. No. 4,706,097 patent. This spring connect structure includes a central locating member having a plurality of cylinders extending integrally therethrough and therefrom to a predetermined distance from each major surface of the central locating member. Cone-shaped tips located at upper ends of the elastomeric deflectable cylinders are inserted into dimples of the flex circuit with a force sufficient to bring the electrical bumps or pads above the dimples into good electrical contact with mating conductive contact pads on the printhead substrate. The volumetric deformation of the elastomeric deflectable cylinders varies substantially linearly as a function of the force applied to the lower ends of these cylinders. This feature enables the vertical displacement of the cylinder walls to be maximized for a given force applied to these cylinder.
The above-described rubber parts present a problem to the user. More specifically, in order to function in the manner described above, the rubber components must be manufactured to a high level of precision. However, precision rubber components are difficult at best to manufacture.